Use the correct directories for non-mod files; use .pw.toml extension

The mods-folder option is now replaced with two new options: meta-folder and meta-folder-base
This allows non-mod files to use the correct directory based on their category; with correct
import of resource packs/etc from CurseForge packs, and the ability to override this behaviour.
To improve the reliability of packwiz metadata file marking (in the index), new files now use .pw.toml
as the extension - any extension can be used, but .pw.toml will now be automatically be
marked as a metafile regardless of folder, so you can easily move metadata files around.
Existing metadata files will still work (as metafile = true is set in the index); though in
the future .pw.toml may be required.

modrinth/install.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"github.com/spf13/viper"
 	"os"
+	"path/filepath"
 	"regexp"
 	"strings"
 
@@ -207,10 +208,14 @@ func installVersion(mod Mod, version Version, pack core.Pack) error {
 		Update: updateMap,
 	}
 	var path string
+	folder := viper.GetString("meta-folder")
+	if folder == "" {
+		folder = "mods"
+	}
 	if mod.Slug != "" {
-		path = modMeta.SetMetaName(mod.Slug, index)
+		path = modMeta.SetMetaPath(filepath.Join(viper.GetString("meta-folder-base"), folder, mod.Slug+core.MetaExtension))
 	} else {
-		path = modMeta.SetMetaName(mod.Title, index)
+		path = modMeta.SetMetaPath(filepath.Join(viper.GetString("meta-folder-base"), folder, mod.Title+core.MetaExtension))
 	}
 
 	// If the file already exists, this will overwrite it!!!